1. Field of the Invention
This invention relates to a sheet folding apparatus for folding a bundle of sheets, and an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus provided with the sheet bundle folding apparatus, and more particularly relates to a sheet bundle folding apparatus for folding a bundle of sheet by a pair of folding rollers and an image forming apparatus provided with the same.
2. Related Background Art
As a sheet bundle folding apparatus mounted on an image forming apparatus such as a printer or a facsimile apparatus, as described, for example, in Japanese Patent Laid-Open Application No. 4-333469. This is provided with two sets of roller pairs, i.e., a pair of pre-fold rollers for folding a bundle of sheets after image formation and a pair of pressing rollers, and a folding bar for pushing the bundle of sheets into the nip between the pair of pre-fold rollers.
In the sheet bundle folding apparatus of such a construction, is shown for example in, the bundle of sheets after image formation is conveyed upwardly of the pair of pre-fold rollers, whereafter as shown in FIG. 17 of the accompanying drawings, the folding bar 1 is protruded toward the pair of pre-fold rollers 2 (e.g. downwardly). When the folding bar 1 is thus protruded downwardly, the central portion of the bundle of sheets 3 is pushed into the nip 2a between the pair of pre-fold rollers 2 rotated in the roll-in direction indicated by an arrow, by the folding bar 1, whereby the bundle of sheets 3 is enfolded from the central portion thereof by the pair of pre-fold rollers 2 and is folded into two.
Thereafter, the bundle of sheets 3 folded into two by the pair of pre-fold rollers 2 is rolled into a pair of pressing rollers 2A disposed below the pair of pre-fold rollers 2, and is further pressed by this pair of pressing rollers 2A and is neatly folded into two.
Now, in such a sheet bundle folding apparatus according to the prior art, the fold height of the folded bundle of sheets 3 and the drive torque for the pair of rollers required for folding are in an inverse proportional relation and therefore, a great drive torque will become necessary if an attempt is made to make the fold height low. For example, to neatly fold a sheet of A3 size (80 g/m.sup.2) into a sheet of A4 size so that the fold height when it is placed on a flat surface may be of the order of 30 mm, a great drive torque of the order of 25 kg/cm becomes necessary as the drive torque for the pressing rollers.
To produce such a great drive torque, a large drive device becomes necessary, but there has been the problem that an attempt to provide such a large drive device leads to the bulkiness of the entire sheet bundle folding apparatus, which in turn leads to the bulkiness of the image forming apparatus.
Also, the apparatus according to the prior art, as described above, is designed such that sheet folding is effected by the two sets of a roller pairs, i.e., the pair of pre-fold rollers and the pair of pressing rollers and therefore, a drive device becomes necessary for each of the roller pairs, and this has led to the problem of complicated structure.